The present invention relates to methods for performing handover procedures in a communication network. More particularly, the present invention relates to methods for accelerating handover procedures in a communication network.
In a conventional wireless communication network, when the distance between communication parties increases or a shadowing effect occurs, the intensity of signals received at a station may decrease, which in turn may degrade the transmission performance of the communication network, especially when the station is located at or near an edge of coverage of the wireless communication network. To extend the coverage of the wireless communication network and improve the transmission rate, one or more relay station (RS) may be deployed to forward signals between a base station (BS or MR-BS) and a mobile station (MS). In some applications, a relay station may be located within a vehicle to provide its attached mobile stations (e.g. cellular phones used by passengers in the vehicle) higher throughput or maintain connectivity to a base station. Such a relay station may be advantageous in that the distance between the mobile stations and the relay station in the vehicle is almost fixed, no matter whether the vehicle moves or not. Furthermore, the almost fixed distance between the mobile stations and the relay station may facilitate reliable and stable channel quality when the vehicle migrates. However, when a passenger using a mobile station is getting off the vehicle, a fast handover from the relay station to an outside base station may be needed. In such a situation, a conventional handover procedure may not be able to perform a handover in time to avoid disconnection.
Some techniques in the prior art have been developed to resolve this problem. An example of the prior art techniques may be found in U.S. Pat. No. 7,031,711 (hereinafter “the '711 patent”) to Samir S. Soliman, entitled “Mobile Communication System With Position Detection To Facilitate Hard Handoff.” In the '711 patent, a positioning equipment such as a GPS (Global Positioning System) may be required, which is embedded within a mobile station to facilitate a hard handover (or handoff) procedure. Furthermore, a database of positions may also be required, which is maintained by the mobile station and a base station so as to provide information to track the corresponding mobile trajectory. As a result, the '711 patent may require an additional positioning device to acquire information on location in order to perform a hard handover.
Another example may be found in U.S. Patent Publication number 2005/0128969 (hereinafter “the '969 application) by Min-ho Lee et. al., entitled “Handover Method and Handover Apparatus.” The '969 application may provide a method and an apparatus for applying measurement calculation to estimate the moving speed of a mobile station so as to reduce handover latency or packet loss rate when the mobile station moves at a relatively high speed. The apparatus may be designed for use in a media access control (MAC) or physical layer for the handover procedure, and may not be able to reduce the latency or increase transmission performance using a higher layer of mechanism in the communication network.
Still another example may be found in IEEE802.16j-06—026r3-P802.16j Baseline Document, wherein methods of handover procedures related to “Mobile-RS” are specified. These methods may be divided into mobile relay station (Mobile-RS) handover with and without preamble change in dealing with interference issues. Both cases (with and without preamble change) may each be further divided into Mobile-RS handover procedures and attached mobile stations (MSs) handover procedures. A basic concept of these methods resides in reusing legacy 802.16e handover procedures in the conditions where the Mobile-RS may act as an interface to facilitate the handover procedures of its attached MSs. However, the MSs may need to handover once the Mobile-RS to which the MSs are attached handover. Furthermore, latency of the handover may not be reduced significantly.
FIG. 1 is a diagram illustrating a communication network 100. The communication network 100 may include a first sub-network 102 and a second sub-network 104. The first sub-network 102 may include a base station 106, relay stations 108c, 108d, 108e and 108f, and mobile stations 110a, 110b, 110c and 110d. The base station 106 may have a coverage 112, while the relay stations 108c to 108f may respectively have coverage 114c to 114f. The second sub-network 104 may include relay stations 108a and 108b with coverage 114a and 114b, respectively. Furthermore, the relay stations 108c, 108d, 108e and 108f may be used for throughput enhancement of the communication network 100, while the relay stations 108a and 108b may be used for coverage extension of the communication network 100.
Usually, relay stations (RSs) in a wireless communication network may be divided into three types by mobility, namely the fixed-RS, nomadic-RS and mobile-RS. The fixed-RS may refer to an RS mounted at a fixed location. The nomadic RS may refer to an RS which is portable but may not provide service until it is positioned. For these types of relay stations, only mobile stations (MSs) may need to deal with mobility management during operation. The fixed-RS or the nomadic-RS may connect with a stable access station (e.g. an MR-BS), which in turn may be a target access station for the attached MSs of the fixed-RS or the nomadic-RS when the MSs are going to move out of the coverage of the original fixed-RS or nomadic-RS.
Unlike the nomadic-RS, the mobile-RS may provide service during its motion or migration. Accordingly, mobility management may be required for the mobile-RSs and their attached MSs. FIG. 2A and FIG. 2B are diagrams illustrating handover in a mobile-RS. Referring to FIG. 2A, a wireless communication network may include a base station (BS) 200, a mobile-RS 202 and a mobile station (MS) 206. The BS 200 has a coverage 208 and the mobile-RS 202 has a coverage 210. The mobile-RS 202 may be located at a vehicle 204 and thus may move with respect to the BS 200 as the vehicle 204 moves. Moreover, the BS 200 and the mobile-RS 202 may originally communicate with a first wireless link 212 and the mobile-RS 202 and the MS 206 may originally communicate with a second wireless link 214. The mobile-RS 202 may continuously send radio signals such as preamble, MAP or data, to its attached MS 206 to maintain connectivity with the attached MS 206. Moreover, the second wireless link 214 between the mobile-RS 202 and the MS 206 may be stable even if the vehicle 204 moves because the attached MS 206 of the mobile-RS 202 moves with the mobile-RS 202. The MS 206 may not require any handover procedures even when the vehicle 204 moves at a relatively high speed as long as the MS 206 stays in the vehicle 204. The attached MS 206 may be always in the coverage 210 of the mobile-RS 202 when in the vehicle 204, and thus may not need any handover to communicate with other stations, which is an advantage of the mobile-RS 202.
However, as the mobile-RS 202 migrates from the BS 200 to another, power transmitted by the mobile-RS 202 may be interfered with other BSs or RSs and thus the overall system performance may be degraded. To resolve the issue of power interference, in one aspect, the BS 200 may configure the mobile-RS 202 to transmit at a suitable frequency (segment) based on the current location of the mobile-RS 202. However, this change in frequency may trigger the attached MS 206 of the mobile-RS 202 to perform a handover procedure in order to re-synchronize itself with the same access station at a different transmitting frequency, which may incur undesirable overhead for the attached MS 206 and cause the mobile-RS 202 inefficient as compared to the case where the MS 206 is connected with the BS 200 directly without relaying through the mobile-RS 202.
In another aspect, the transmitting power of the mobile-RS 202 may be limited to resolve the issue of power interference. Since the mobile-RS 202 may have a confined radio coverage related to the vehicle 204, the MS 206 may be allowed to communicate with the mobile-RS 202 only when the MS 206 is in the vehicle 204. However, the power of signals of the mobile-RS 202 may decay drastically as a passenger with the MS 206 debarks the vehicle 204. Referring to FIG. 2A, a link 214 between the MS 206 out of the coverage 210 and the mobile-RS 202 may degrade relatively fast. As a result, the MS 206 may lose its connectivity for lack of sufficient response time to perform a handover to switch from the mobile-RS 202 to another station.
It may therefore be desirable to have a handover method for a wireless multi-hop relay communication system for improving the transmission efficiency and capacity of the system.